The present invention relates to circuitry for switching electrical current at high speed.
In many applications, and particularly in electronic circuit testing, there is a need to control current flow through a load so that the current can be turned on and off at very high speeds. Such current switching has traditionally been accomplished by using specially selected transistors, SCRs, or other circuit elements. However, these components are limited in their speed of switching, their current capability, and their input/output impedance characteristics. Prior art systems tend to be optimized for either a high switching speed or continuous operation stability in the conductive (on) state, but not both. Prior art switching techniques that have been optimized for high speed switching suffer from poor reliability when used in continuous duty situations because the components tend to overheat and fail. Conversely, prior art switching techniques that have been optimized for stable "on" operation suffer from degraded switching characteristics.
The foregoing shortcomings are particularly evident in prior art circuits designed to test F16 Low Noise Assemblies. To effectively test such circuits, the high speed switching (approximately 3nS.) of high level currents (70-80mA) into a low impedance ground referenced load is necessary, while maintaining a high input impedance (15,000 to 24,000 ohms) and a low output impedance (12-50 ohms). Continuous duty capability is necessary, as well as the ability to withstand short circuit load conditions. Prior art testing circuits attempted to meet this criteria by means of a transistor operated constant current source that was also a switching transistor. The power transistor in the constant current source/switch regulated the current output when the 70-80 milliamperes current was being conducted through the Low Noise Assembly. While such prior art testing circuits were able to switch currents of this magnitude at moderately high speeds, this could only be accomplished by hand selection of the resistors and other components to adjust the output current level of the circuit. Additionally, the thermal stresses to which the switching transistor was subjected as a result of the continuous operation of the testing circuit created instabilities in its switching characteristics which, in the short run, affected its reliability and in the long run necessitated its replacement, along with the replacement of the surrounding precision resistors. These same thermal stresses that adversely affected the reliability of the transistor also caused undesired variations in the output current level conducted to the load. While the Low Noise Assemblies could be successfully tested by these prior art testing circuits despite the instabilities caused by the thermal stresses on the current control and switching transistor, the resulting inconsistencies in the switching characteristics and current level necessitated programmed delays, calibrations and corresponding adjustments to the test results to compensate for inaccuracies. Accordingly, there is a need for a high speed current switch which can switch currents on the order of 70 to 80 milliAmperes within 3 nanoseconds into a low-impedance ground referenced load of 0-50 Ohms while maintaining a relatively high input impedance of 15 to 24 kiloOhms and low output impedance of 12 to 50 Ohms with continuous duty capability even under short circuit load conditions.